1. Field of the Invention
This invention relates to electric power transmission system protection apparatus.
2. Description of the Related Art
Such apparatuses frequently employ so-called distance relays to determine whether a fault on the system is within a predetermined distance of a monitoring point where the relay is located. The present invention is particularly concerned with distance relays which respond to single phase to ground faults, a separate such relay being normally provided for each phase of polyphase power transmission system.
In a well-known form of such a relay the determination is made by comparing the phases of voltages derived from measurements of the system voltage and current at the monitoring point under fault conditions. For example, referring to FIG. 1, in the so-called mho characteristic relay of the PRIOR ART the phases of quantities V-IZ and V.sub.pol are compared where:
V is the phase voltage at the monitoring point; PA1 I is the phase current at the monitoring point; PA1 Z is a replica impedance which determines the setting of the relay i.e. the predetermined distance or reach; and PA1 V.sub.pol is V&lt;-90.degree..
Since the quantities V-IZ and V are necessarily at 90.degree., and hence the quantities V-IZ and V.sub.pol are necessarily in phase when the meeting point of the vectors V and V-IZ lies on a circle having IZ as diameter, by determining, using a phase comparator, whether V-IZ leads or lags V.sub.pol it can be determined whether the fault is beyond or within the distance for which the tip of the vector V lies on the circle.
Whilst such a relay has many applications, it suffers from the disadvantage that its reach depends on the impedance presented by the fault, and in particular, decreases significantly for faults with a high resistance e.g. such as may occur for faults near the monitoring point when the ground contact resistance is high or the lines of the system are supported on towers with high footing resistance.
To overcome this problem it is known in the PRIOR ART to use a distance relay having a quadrilateral characteristic such as that shown in FIG. 2. With this relay each side of the quadrilateral characteristic is independently defined by a separate phase comparator, the four sides together defining the reach of the relay in the same way as the circle defines the reach of a mho characteristic relay.
Thus the top or "reactance" line of the quadrilateral is defined by a comparator which determines the relative phase of quantities A.sub.1 and B.sub.1 which are chosen according to the required slope and position of the line. Similarly the right hand "resistance" line of the quadrilateral is defined by a comparator responsive to quantities A.sub.2 and B.sub.2, the left hand "resistance" line of the quadrilateral is defined by a comparator responsive to quantities A.sub.3 and B.sub.3 and the bottom "directional" line of the quadrilateral is defined by a comparator responsive to quantities A.sub.4 and B.sub.4.
One difficulty which arises with a quadrilateral characteristic relay is that if the relay is optimised for single-phase to ground fault operation, when a two-phase to ground fault occurs the top side, i.e. reactance line, of the characteristic changes slope causing the relay to overreach or underreach.
To overcome this problem it has been proposed that the single phase to ground phase comparators be blocked from operation if a phase to phase fault comparator or a two-phase to ground fault comparator indicates a fault. However, this suffers from so-called `race` problems, i.e. it necessarily requires the phase to phase fault comparator to indicate a fault first for satisfactory operation.